1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device containing MOSFETs and a method of fabricating the same. More particularly, the present invention relates to a semiconductor integrated circuit device including MOSFETs having metal silicide films on the surface of source and drain regions and a method of fabricating the same.
2. Description of the Prior Art
In semiconductor integrated circuit devices, reduction in the element dimension has been progressing rapidly due to increase in the device density and increase in the operating speed. Further, in semiconductor integrated circuit devices including MOSFETs, suppression of the short channel effect of the MOSFETs is also required. For this purpose, source and drain regions are made shallow. However, making the junction of the source-drain diffused layer shallow brings about an increase in the sheet resistance thereof. Since the source and drain diffused layers and the gate electrodes are used as a part of the wirings, the increase in the sheet resistance of the diffused layers and the reduction in the size of the gate electrodes causes an increase in the wiring resistance to lower the operating speed of the circuit.
The increase in the sheet resistance of the gate electrode due to the reduction in the size thereof can be prevented by using suitable material for the gate electrodes. In recent years, in place of a gate electrode only made of a polycrystalline silicon film (the so-called silicon gate electrode), a gate electrode having a structure of laminating a metal silicide film on a polycrystalline silicon film (the so-called polycide structure) or a gate electrode only made of a metal silicide film has been used. Further, a gate electrode made of a refractory metal has been employed.
On the other hand, concerning the sheet resistance, a structure for reducing the sheet resistance of regions was disclosed in IEDM Tech. Dig., 1982, pp. 714-717 by C. K. Lau et al. According to this structure, a metal film is deposited on the surface of a diffused layer (or region) formed in the surface portion of a silicon substrate, a silicification reaction is induced by heat treatment between silicon of the diffused layer and the metal film, the metal film left unreacted being thereafter removed. Thus, a metal silicide film is formed in a self-aligned fashion with respect to the diffused layer. The structure of the MOSFET obtained thus is called "salicide" (abbreviation for self-aligned silicide). The sheet resistance of the diffused layer is thereby converted from several tens to one hundred and several tens of ohms/square (.OMEGA./.quadrature.) into several ohms/square (.OMEGA./.quadrature.).
However, a MOSFET with the silicide structure has resistance against electrostatic discharge (called hereinafter "ESD resistance") that is remarkably smaller than the ESD of a MOSFET not having the silicide structure, as reported by K. L. Chen, et al, in IEDM Tech. Dig., 1986, pp. 484-487. According to the present inventor, moreover, ESD resistance of a MOSFET with the silicide structure is about one-third of that of a MOSFET not having the silicide structure.
The reason for this can be considered as follows. That is, the silicide structure reduces the resistance of the diffused layer itself. When a discharge current due to static electricity flows into the diffused layer (in particular the drain diffused layer) of a MOSFET, therefore, the current tends to concentrate at the portion of the layer just under the end part of the gate electrode. Consequently, a local thermal breakdown is apt to be generated in the gate insulating film in the vicinity of the gate electrode end part. The MOSFET is thereby destroyed. Thus, MOSFETs with the silicide structure in buffer circuits such as an input buffer circuit, an output buffer circuit and an I/O buffer circuit deteriorate EDS resistance of an integrated circuit device.